From the Q&A

With TJ Byers

Tolerance Makes Cents

Question:

I have a question about the meaning of parts specifications. Let's say you buy 36-volt zener diodes at 5% tolerance. Does it mean that three-sigma from 36 volts is 5% and that all the parts bought will fall within that tolerance? I'm looking to see what the sigma is so as to properly tune the design.

Fred Sprague
via Internet

Answer:

Electronic components are rated on a fixed deviation of plus and minus a fixed value, not three-sigma (which is a mathematical deviation curve). For example, a 36-volt zener diode with a 5% tolerance means that the voltage of the zener is guaranteed to be ±1.8 volts of the standard value; in your example, the zener voltage is in the range of 34.2 to 37.8 volts. The actual values tend to cluster by the batch run, with one batch having a peak around one value and another batch centering around a different value. Temperature also affects the voltage of a zener, which is why the tolerance is always rated at a specific temperature. Typical temperature ranges are 0° C to +70° C (commercial) and -55° C to +125° C (military).

Tolerance is not an indication of poor manufacturing. Closer tolerances can be achieved, but at greater expense. A zener (or resistor) with a 10% percent tolerance costs less to produce than one with a 5% tolerance.